In the current manual, connection to the AC1IN suggests the generator. However, this should be different per application, f.e. on a boat AC1IN could be better the Shore-power, than the auto start/stop of the generator can be used in the (CC)GX.

So: "AC1IN connected to the most stable AC source".

From the current manual:

AC-in-1 (see appendix A) If AC voltage is present on these terminals, the Quattro will use this connection. Generally a generator will be connected to ACin-1. The AC-in-1 input must be protected by a fuse or magnetic circuit breaker rated at 50A or less, and cable crosssection must be sized accordingly. If the input AC supply is rated at a lower value, the fuse or magnetic circuit breaker should be down sized accordingly.